Recently, an electron beam drawing apparatus is used to draw a fine pattern on a sample such as a semiconductor wafer or mask substrate. This apparatus suffers the influence of a so-called proximity effect in which a pattern becomes thick or thin owing to backward scattering electrons.
One of effective methods of correcting the proximity effect is a dosage correction method. As the method of determining the optimal dosage, there are adopted method (a) using a matrix (M. Parikhh, J. App. Phys. 19, p. 4371, p. 4378, p. 4383 (1979)), and method (b) using a simple approximate solution formula (e.g., J. M. Parkovich, Journal of Vacuum Science & Technology B4, p. 159 (1986)).
According to method (a), the relationship between the dosage and the sensitivity at each position is expressed by a matrix, and the inverse matrix of this matrix is calculated to obtain an optimal dosage at each position. The advantage of this method is that an accurate optimal dosage can be obtained by sufficiently decreasing the size of a figure for which a dosage is set. The disadvantage is a long calculation time. This method requires several hundred to several thousand hours to correct the proximity effect for all direct writing LSI chips.
According to method (b), an approximate value D′ of an optimal dosage is calculated by, e.g., the following formulae (1) and (2):D′=C/(½+ηU)  (1)U=(1/π)∫exp{−(x−x′)2−(y−y′)2}dx′dy′  (2)
where C is a constant, and η is the ratio of a resist sensitivity by forward scattering of an electron beam to that by backward scattering. Letting (x,y) be a dosage evaluation point, the integral range of the parameter U is a rectangle present within a circle having a radius about two or three times the backward scattering radius using (x,y) as the center, or a rectangle overlapping even part of the circle. However, even the use of the approximate solution formulae requires several hours to correct the proximity effect for all direct writing LSI chips.
In this manner, the conventional methods require at least several hours for a proximity effect correction calculation time. On the other hand, η, the backward scattering radius, and the like as parameters of proximity effect correction calculation change depending on a resist applied to a target wafer, the film material of a wafer surface, or the like, so that parameters (η, backward scattering radius, and the like) for achieving an optimal proximity effect must be obtained. For this purpose, the step of changing parameters to recalculate proximity effect correction, and the step of actually exposing and evacuating a wafer using the recalculated proximity effect correction are repeated. This occupies an electron beam drawing apparatus for several ten hours in order to only determine parameters, which decreases the availability of the electron beam drawing apparatus.